Posted on: January 20, 2020 at 12:19 pm

For decades, people undergoing radiotherapy, which is used to treat cancer, have reported a bizarre phenomenon: Seeing flashes of light in their eyes, even when their eyes are closed.


Patients documented in the medical literature have described a ‘‘ray of blue light” and ‘‘seeing a blue neon light”, sometimes accompanied by a “white smell” during the delivery of radiation, lasting for a fraction of a second. There have been several theories for why this could be happening, including retinal pigments inside patients’ eyes being stimulated during the therapy, or that Cherenkov light or Cherenkov radiation – the same effect that makes nuclear reactors glow blue when they’re underwater – is produced inside the eyeball itself.

Now scientists have captured this strange light for the first time, producing the first photographic evidence that the phenomenon is in fact Cherenkov light.

Cherenkov light inside a patient’s eye. Lesley Jarvis

Cherenkov light is electromagnetic radiation that’s emitted when a charged particle passes through matter at a greater speed than the speed of light within that medium, producing an effect like a sonic boom, which occurs when, for example, a plane travels faster than the speed of sound.

To be very clear, nothing is breaking the speed of light as it is in a vacuum, rather the speed of light as it travels through a medium such as water.

This video from KInzenir on Twitter shows blue light being produced when a nuclear reactor is placed underwater.

Finding Cherenkov light in the human eyeball was no easy task.


A team of researchers from Dartmouth’s and Dartmouth-Hitchcock’s Norris Cotton Cancer Center – who published their work in the International Journal of Radiation Oncology – used technology called a CDose camera imaging system, specially designed to view light emissions from biological systems.

Our newest data is exciting because for the first time, light emission from the eye of a patient undergoing radiotherapy was captured,” co-lead Irwin Tendler of Dartmouth College said in a statement. “This data is also the first instance of evidence directly supporting that there is enough light produced inside the eye to cause a visual sensation and that this light resembles Cherenkov emission.

As the radiation beam passes through the eye, light is generated within the vitreous fluid. Our real-time data rigorously showed that the amount of light produced is sufficient to elicit a visual sensation – a topic that has been debated in the literature. By analyzing the spectral composition, we also show that this emission can be classified as Cherenkov light – again, another contested point in the literature.”

The team hopes that the research and imaging system could be used to help improve radiation therapy. They suggest that monitoring light emissions from patients’ eyes could help show medics whether the radiation has hit the intended target, for instance. The explanation for the phenomenon could also help reassure patients who experience the strange phenomenon of flashes of light coming from within their own eyes.

Shared with permission from our friends at IFL Science.

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